1. Field of the Invention
The present invention relates to a method and apparatus for laser cutting sheet material and especially for laser cutting composite sheet material. An inert gas shield is provided on both sides of the sheet material at the location of the cut to prevent oxidation thereof. Additionally, the sheet material is accurately positioned so as to lie at the focal point of the laser beam when the cut is performed.
2. Description of the Prior Art
It has become customary to form on dielectric sheet material printed circuits used in a variety of electronic components. One type of material which has come to be widely used are composites such as glass fiber cloth as a matrix embedded in or impregnated with an epoxy resin. In its uncured state, such composite materials are often times referred to as "prepreg". Prepreg is flexible and in sheet form is capable of being wound onto a roll.
In present day practice, the prepreg may typically be cut into rectangular slabs, or other appropriate shapes, then cured into hardened boards with copper sheets applied as exterior surfaces thereof, after which the printed circuitry is configured on the outer surface thereof. However, there are a number of problems inherent in the mechanical cutting process which has been used heretofore. Even with fresh cutting blades, frayed or jagged edges of the cloth matrix and tearing of the resin layer customarily result and such results are compounded with increased age, and therefore dullness, of the blades. Large amounts of dust also result from the mechanical cutting process and are deposited on the surfaces of the individual boards. Migration of this dust to the top surface of the copper sheets during subsequent lamination processes requires its removal before the subsequent etching process resulting in formation of the electrical circuit thereon. This causes another step to be added to the circuit board making process with resultant increase in cost. Additionally, the cutting blades must be frequently sharpened or replaced and the cutting mechanism generally requires a substantial degree of maintenance especially for the moving components. Another drawback of the mechanical cutting operation is the limited flexibility of such a system for cutting prepreg boards of various sizes.
Because of these drawbacks of mechanical cutting systems, the cutting of prepreg with a laser has been considered as a viable alternative. Lasers have been used for some time to cut a variety of materials including metals, wood, fabric and paper. However, to date, there has been no widespread usage of lasers for cutting composite materials. Typical of the prior art are the U.S. Pat. No. 3,597,578 issued Aug. 3, 1971 to Sullivan et al and 3,604,890 issued Sept. 14, 1971 to Mullaney et al. Both of these disclose the cutting of various materials by means of concentrated laser beams. Improvements on such basic patents are disclosed, for example, in U.S. Pat. No. 3,679,863 issued July 25, 1972 to Houldcroft et al. According to this patent, a jet of gas such as air, argon, or nitrogen carrying an atomized stream of a liquid coolant such as water is directed onto the moving region of the workpiece at which the laser beam is concentrated. As recited in the patent, this reduces the spread of charring or tar deposit from the cut along the upper and lower surfaces of the cut portions and also reduces the smoke and smell to negligible proportions. A further improvement is provided in U.S. Pat. No. 4,002,877 issued Jan. 11, 1977 to Banas. According to this disclosure, a stream of liquid coolant such as water is directed onto the material vaporization region to absorb energy from the outer region of the cut area, said to prevent adhesion of adjacent layers, globularization, singeing, or permitting other undesired effects.
However, composite materials add even further problems because they employ at least two types of materials having different characteristics. For example, the prepreg material already discussed comprises a matrix of glass in fiber form supported in a layer of uncured epoxy resin. Since each of these materials oxidizes at a different temperature, use of lasers which have been adequate to cut other materials has been inadequate for cutting the prepreg material. This is for the reason that the temperatures necessary to cut the glass fibers would cause the epoxy resin to decompose and melt resulting in undesirable flow of the fibers within the resin and charring and tearing of the resin layer.